Rectifier in low gain wireless power transfer system

ABSTRACT

An apparatus includes a receiver coil configured to be magnetically coupled to a transmitter coil, and a rectifier circuit coupled to two terminals of the receiver coil, wherein in response to a high system gain of the apparatus, the rectifier circuit is configured as a half-bridge rectifier, and in response to a low system gain of the apparatus, the rectifier circuit is configured as a full-bridge rectifier.

PRIORITY CLAIM

This application claims priority to Chinese Patent Application No.202110345121.X, filed on Mar. 30, 2021, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates to a rectifier circuit in a low gainwireless power transfer system.

BACKGROUND

As technologies further advance, wireless power transfer has emerged asan efficient and convenient mechanism for powering or charging batterybased mobile devices such as mobile phones, tablet PCs, digital cameras,MP3 players and/or the like. A wireless power transfer system typicallycomprises a primary side transmitter and a secondary side receiver. Theprimary side transmitter is magnetically coupled to the secondary sidereceiver through a magnetic coupling. The magnetic coupling may beimplemented as a loosely coupled transformer having a primary side coilformed in the primary side transmitter and a secondary side coil formedin the secondary side receiver.

The primary side transmitter may comprise a power conversion unit suchas a primary side of a power converter. The power conversion unit iscoupled to a power source and is capable of converting electrical powerto wireless power signals. The secondary side receiver is able toreceive the wireless power signals through the loosely coupledtransformer and convert the received wireless power signals toelectrical power suitable for a load.

As power consumption has become more important, there may be a need forhigh power density and high efficiency wireless power transfer systems.In a high power wireless transfer system, a larger current output leadsto a temperature rise in the receiver coil of the wireless powertransfer system. Such a temperature rise causes poor system efficiency.In order to overcome this drawback, a low inductance receiver coil maybe employed to reduce the temperature rise in the receiver coil.However, the receiver having a low inductance receiver coil may be usedin a variety of applications such as a low power application (e.g., thepower of the wireless power transfer system is less than 10 W). In thelow power application, the receiver having a low inductance receivercoil is not compatible with a low power transmitter (e.g., a transmitterhaving a low input voltage). It would be desirable to have a highperformance receiver exhibiting good behaviors. For example, a highefficiency receiver is compatible with a variety of operatingconditions.

SUMMARY

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by preferred embodiments ofthe present disclosure which provide a high efficiency rectifier circuitcompatible with a variety of operating conditions.

In accordance with an embodiment, an apparatus comprises a receiver coilconfigured to be magnetically coupled to a transmitter coil, and arectifier circuit coupled to two terminals of the receiver coil, whereinin response to a high system gain of the apparatus, the rectifiercircuit is configured as a half-bridge rectifier, and in response to alow system gain of the apparatus, the rectifier circuit is configured asa full-bridge rectifier.

In accordance with another embodiment, a method comprises determining asystem gain of a wireless power transfer system comprising a transmittercoil, a receiver coil and a rectifier circuit coupled to the receivercoil, in response to a high system gain application of the wirelesspower transfer system, configuring the rectifier circuit as ahalf-bridge rectifier, and in response to a low system gain applicationof the wireless power transfer system, configuring the rectifier circuitas a full-bridge rectifier.

In accordance with yet another embodiment, a system comprises atransmitter coil coupled to an input power source through a transmittercircuit, a receiver coil magnetically coupled to the transmitter coil,and a rectifier circuit coupled to the receiver coil, wherein inresponse to a low input voltage of the system, the rectifier circuit isconfigured as a half-bridge rectifier to boost an output voltage of thesystem.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter which form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present disclosure. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the disclosure as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a wireless power transfer systemin accordance with various embodiments of the present disclosure;

FIG. 2 illustrates a schematic diagram of the wireless power transferdevice shown in FIG. 1 in accordance with various embodiments of thepresent disclosure;

FIG. 3 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a first phase of the first configuration of thereceiver in the high system gain application in accordance with variousembodiments of the present disclosure;

FIG. 4 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a second phase of the first configuration ofthe receiver in the high system gain application in accordance withvarious embodiments of the present disclosure;

FIG. 5 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a first phase of the second configuration ofthe receiver in the high system gain application in accordance withvarious embodiments of the present disclosure;

FIG. 6 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a second phase of the second configuration ofthe receiver in the high system gain application in accordance withvarious embodiments of the present disclosure;

FIG. 7 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a first phase of the low system gainapplication in accordance with various embodiments of the presentdisclosure;

FIG. 8 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a second phase of the low system gainapplication in accordance with various embodiments of the presentdisclosure; and

FIG. 9 illustrates a flow chart of controlling the receiver shown inFIG. 2 in accordance with various embodiments of the present disclosure.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the variousembodiments and are not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent disclosure provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the disclosure, and do not limit the scope of the disclosure.

The present disclosure will be described with respect to preferredembodiments in a specific context, namely a rectifier circuit compatiblewith different operating conditions of a wireless power transfer system.The invention may also be applied, however, to a variety of powerconversion devices of the wireless power transfer system. Hereinafter,various embodiments will be explained in detail with reference to theaccompanying drawings.

FIG. 1 illustrates a block diagram of a wireless power transfer systemin accordance with various embodiments of the present disclosure. Thewireless power transfer system 100 comprises a power converter 104 and awireless power transfer device 101 connected in cascade between an inputpower source 102 and a load 114. The wireless power transfer device 101includes a transmitter 110 and a receiver 120. As shown in FIG. 1 , thetransmitter 110 comprises a transmitter circuit 107 and a transmittercoil L1 connected in cascade. The input of the transmitter circuit 107is coupled to an output of the power converter 104. The receiver 120comprises a receiver coil L2 and a rectifier circuit 112 connected incascade. The output of the rectifier circuit 112 is coupled to the load114.

The transmitter 110 is magnetically coupled to the receiver 120 througha magnetic field when the receiver 120 is placed near the transmitter110. A loosely coupled transformer 115 is formed by the transmitter coilL1, which is part of the transmitter 110, and the receiver coil L2,which is part of the receiver 120. As a result, power may be transferredfrom the transmitter 110 to the receiver 120.

In some embodiments, the transmitter 110 may be inside a charging pad.The transmitter coil is placed underneath the top surface of thecharging pad. The receiver 120 may be embedded in a mobile phone. Whenthe mobile phone is place near the charging pad, a magnetic coupling maybe established between the transmitter coil and the receiver coil. Inother words, the transmitter coil and the receiver coil may form aloosely coupled transformer through which a power transfer occursbetween the transmitter 110 and the receiver 120. The strength ofcoupling between the transmitter coil L1 and the receiver coil L2 isquantified by the coupling coefficient k. In some embodiments, k is in arange from about 0.05 to about 0.9.

In some embodiments, after the magnetic coupling has been establishedbetween the transmitter coil L1 and the receiver coil L2, thetransmitter 110 and the receiver 120 may form a power system throughwhich power is wirelessly transferred from the input power source 102 tothe load 114.

The input power source 102 may be a power adapter converting a utilityline voltage to a direct-current (dc) voltage. Alternatively, the inputpower source 102 may be a renewable power source such as a solar panelarray. Furthermore, the input power source 102 may be an energy storagedevice such as rechargeable batteries, fuel cells and/or the like.

The load 114 represents the power consumed by the mobile device (e.g., amobile phone) coupled to the receiver 120. Alternatively, the load 114may refer to a rechargeable battery and/or batteries connected inseries/parallel, and coupled to the output of the receiver 120.

The transmitter circuit 107 may comprise primary side switches of afull-bridge converter according to some embodiments. Alternatively, thetransmitter circuit 107 may comprise the primary side switches of otherconverters such as a half-bridge converter, a push-pull converter andthe like.

It should be noted that the converters described above are merelyexamples. One having ordinary skill in the art will recognize othersuitable power converters such as class E topology based powerconverters (e.g., a class E amplifier), may alternatively be used.

The transmitter circuit 107 may further comprise a resonant capacitor.The resonant capacitor and the magnetic inductance of the transmittercoil may form a resonant tank. Depending on design needs and differentapplications, the resonant tank may further include a resonant inductor.In some embodiments, the resonant inductor may be implemented as anexternal inductor. In alternative embodiments, the resonant inductor maybe implemented as a connection wire.

The receiver 120 comprises the receiver coil L2 magnetically coupled tothe transmitter coil L1 after the receiver 120 is placed near thetransmitter 110. As a result, power may be transferred to the receivercoil and further delivered to the load 114 through the rectifier circuit112. The receiver 120 may comprise a secondary resonant capacitor.

The rectifier circuit 112 converts an alternating polarity waveformreceived from the output of the receiver coil L2 to a single polaritywaveform. In some embodiments, the rectifier circuit 112 is a full-wavebridge formed by switching elements such as n-type metal oxidesemiconductor (NMOS) transistors.

Furthermore, the rectifier circuit 112 may be formed by other types ofcontrollable devices such as metal oxide semiconductor field effecttransistor (MOSFET) devices, bipolar junction transistor (BJT) devices,super junction transistor (SJT) devices, insulated gate bipolartransistor (IGBT) devices, gallium nitride (GaN) based power devicesand/or the like.

The power converter 104 is coupled between the input power source 102and the input of the wireless power transfer device 101. Dependingdesign needs and different applications, the power converter 104 maycomprise many different configurations. In some embodiments, the powerconverter 104 may be a non-isolated power converter such as a buckconverter. In some embodiments, the power converter 104 may beimplemented as a linear regulator. In some embodiments, the powerconverter 104 may be an isolated power converter such as a forwardconverter.

The implementation of the power converter 104 described above is merelyan example, which should not unduly limit the scope of the claims. Oneof ordinary skill in the art would recognize many variations,alternatives, and modifications.

In operation, the wireless power transfer system 100 may be configuredto operate in a low system gain application. More particularly, when thewireless power transfer system 100 is configured to transfer a largeamount of power, the system gain is lowered to reduce the coiltemperature, thereby improving efficiency. In the low system gainapplication, the maximum power transferred between the transmitter andthe receiver is in a range from about 40 W to about 80 W. On the otherhand, the wireless power transfer system 100 may be configured tooperate in a high system gain application. More particularly, when thewireless power transfer system 100 is configured to transfer a smallamount of power, or the input voltage of the wireless power transfersystem is low, the system gain of the wireless power transfer system 100is boosted so as to generate a high output voltage suitable for the loadcoupled to the wireless power transfer system. In the high system gainapplication, the maximum power transferred between the transmitter andthe receiver is in a range from about 5 W to about 10 W. It should benoted that the power levels of the high gain application and the lowgain application described above are merely examples, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Forexample, depending on different applications and design needs, the powertransferred between the transmitter and the receiver in the low gainapplication may be greater than 120 W. Furthermore, the powertransferred between the transmitter and the receiver in the high gainapplication may be in a range from about 30 W to about 40 W.

In some embodiments, the rectifier circuit 112 is configured as either ahalf-bridge rectifier or a full-bridge rectifier so that the receiver120 is compatible with different applications. More particularly, whenthe wireless power transfer system 100 is configured to transfer a largeamount of power, the receiver 120 and the transmitter 110 form a lowgain wireless power transfer system. In some embodiments, the gainbetween the receiver and the transmitter is about 0.5. For example, whenthe input voltage of the transmitter is about 20 V, the output voltageof the receiver is about 10 V. It should be noted that the voltagelevels described above are merely examples, which should not undulylimit the scope of the claims.

In the low gain wireless power transfer system, the rectifier circuit112 is configured as a full-bridge rectifier. The power is transferredbetween the transmitter coil L1 and the receiver coil L2. The receivercoil L2 is smaller than the transmitter coil L1. More particularly, thereceiver coil L2 is of a small inductance in comparison with thetransmitter coil L1. Such a small inductance coil helps to reduce theresistance of the receiver coil, thereby reducing the thermal stress onthe receiver 120.

On the other hand, when the wireless power transfer system 100 isconfigured to transfer a small amount of power, the rectifier circuit112 is configured as a half-bridge rectifier. The half-bridge rectifierfunctions as a voltage booster for increasing the output voltage of thewireless power transfer system 100. The voltage booster is of a gainfrom about 1.3 to about 2. The voltage booster helps to boost the gainof the wireless power transfer system so that the wireless powertransfer system has a normal gain. In some embodiments, the normal gainbetween the receiver and the transmitter is about 0.9.

FIG. 2 illustrates a schematic diagram of the wireless power transferdevice shown in FIG. 1 in accordance with various embodiments of thepresent disclosure. Referring back to FIG. 1 , the wireless powertransfer device 101 comprises a transmitter circuit 107, a transmittercoil L1, a receiver coil L2 and a rectifier circuit 112.

The transmitter circuit 107 is implemented as a full-bridge as shown inFIG. 2 . Throughout the description, the transmitter circuit 107 may bealternatively referred to as the full-bridge 107. The transmittercircuit 107 includes four switching elements, namely Q11, Q12, Q13 andQ14. As shown in FIG. 2 , the switching elements Q11 and Q12 areconnected in series between an input voltage bus VIN and ground. Theinput voltage bus VIN is connected to the output of the power converter104 shown in FIG. 1 . Likewise, the switching elements Q13 and Q14 areconnected in series between the input voltage bus VIN and ground. Thecommon node of the switching elements Q11 and Q12 is coupled to a firstinput terminal of the transmitter coil L1 through the transmitterresonant capacitor C1. The common node of the switching elements Q13 andQ14 is coupled to a second input terminal of the transmitter coil L1.

According to some embodiments, the switching elements Q11, Q12, Q13 andQ14 are implemented as MOSFET or MOSFETs connected in parallel, anycombinations thereof and/or the like. According to alternativeembodiments, the switching elements (e.g., switch Q11) may be aninsulated gate bipolar transistor (IGBT) device. Alternatively, theprimary switches can be any controllable switches such as integratedgate commutated thyristor (IGCT) devices, gate turn-off thyristor (GTO)devices, silicon controlled rectifier (SCR) devices, junction gatefield-effect transistor (JFET) devices, MOS controlled thyristor (MCT)devices, gallium nitride (GaN) based power devices and/or the like.

It should be noted that while the example throughout the description isbased upon a full-bridge converter (e.g., full-bridge 107 shown in FIG.2 ), the implementation of the transmitter circuit 107 shown in FIG. 2may have many variations, alternatives, and modifications. For example,half-bridge converters, push-pull converters, class E based powerconverters (e.g., a class E amplifier) may be alternatively employed.Furthermore, an inductor-inductor-capacitor (LLC) resonant converter maybe formed when the transmitter coil L1 is tightly coupled with thereceiver coil L2 in some applications.

In sum, the full-bridge 107 illustrated herein is limited solely for thepurpose of clearly illustrating the inventive aspects of the variousembodiments. The present invention is not limited to any particularpower topology.

It should further be noted that while FIG. 2 illustrates four switchesQ11, Q12, Q13 and Q14, various embodiments of the present disclosure mayinclude other variations, modifications and alternatives. For example, aseparate capacitor may be connected in parallel with each switch of thefull-bridge 107. Such a separate capacitor helps to better control thetiming of the resonant process of the full-bridge 107.

The outputs of the receiver coil L2 are coupled to the load (shown inFIG. 1 ) through the receiver resonant capacitor C2, the rectifiercircuit 112 and an output capacitor. The rectifier circuit 112 convertsan alternating polarity waveform received from the outputs of thereceiver coil L2 to a single polarity waveform. The receiver resonantcapacitor C2 helps to achieve soft switching for the wireless powertransfer system.

The rectifier circuit 112 includes four switches, namely Q21, Q22, Q23and Q24. As shown in FIG. 2 , the switches Q21 and Q22 are connected inseries between the output terminal (Vo) of the wireless power transfersystem and ground. Likewise, the switches Q23 and Q24 are connected inseries between the output terminal Vo and ground. As shown in FIG. 2 ,the common node of the switches Q21 and Q22 is coupled to a firstterminal of the receiver coil L2 through the receiver resonant capacitorC2. The common node of the switches Q23 and Q24 is coupled to a secondterminal of the receiver coil L2.

According to some embodiments, the switches Q21, Q22, Q23 and Q24 areimplemented as MOSFET or MOSFETs connected in parallel, any combinationsthereof and/or the like. According to alternative embodiments, theswitching elements (e.g., switch Q21) may be an IGBT device.Alternatively, the primary switches can be any controllable switchessuch as IGCT devices, GTO devices, SCR devices, JFET devices, MCTdevices, GaN based power devices and/or the like.

It should be noted that while FIG. 2 illustrates four switches Q21, Q22,Q23 and Q24, various embodiments of the present disclosure may includeother variations, modifications and alternatives. For example, aseparate capacitor may be connected in parallel with each switch of therectifier circuit. Such a separate capacitor helps to better control thetiming of the resonant process of the rectifier circuit.

In operation, the wireless power transfer system may be used in both thehigh system gain application and the low system gain application. In thehigh system gain application, two different configurations areapplicable to the receiver. In a first configuration of the receiver,the second switch Q22 is configured as an always-on switch. Therectifier circuit 112 is configured to operate in two different phasesin response to the first configuration of the receiver. In the twodifferent phases of the first configuration of the receiver, thereceiver coil L2, the receiver resonant capacitor C2, and switchesQ21-Q24 form a half-bridge rectifier. A sum of a voltage across thereceiver coil L2 and a voltage across the receiver capacitor C2 isapplied to a load coupled to the apparatus through the second switch Q22and the third switch Q23. The detailed operating principle of these twophases will be discussed below with respect to FIGS. 3-4 .

In a second configuration of the receiver, the fourth switch Q24 isconfigured as an always-on switch. The rectifier circuit is configuredto operate in two different phases in response to the secondconfiguration of the receiver. In the two different phases of the secondconfiguration of the receiver, the receiver coil L2, the receiverresonant capacitor C2, and switches Q21-Q24 form a half-bridgerectifier. A sum of a voltage across the receiver coil L2 and a voltageacross the receiver capacitor C2 is applied to a load coupled to theapparatus through the fourth switch Q24 and the first switch Q21. Thedetailed operating principle of these two phases will be discussed belowwith respect to FIGS. 5-6 .

In the low system gain application, the rectifier circuit 112 isconfigured to operate in two different phases. The detailed operatingprinciple of these two phases will be discussed below with respect toFIGS. 7-8 .

FIG. 3 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a first phase of the first configuration of thereceiver in the high system gain application in accordance with variousembodiments of the present disclosure. In some embodiments, the receivershown in FIG. 2 is configured to receive a small amount of power. Thetransmitter is a low power transmitter. For example, the powertransferred between the transmitter and the receiver is in a range fromabout 5 W to about 10 W. The input voltage of the transmitter is about10 V. In order to be compatible with the low power transmitter, the gainof the receiver has to be increased accordingly. As shown in FIG. 3 ,the switch Q22 is configured as an always-on switch during the highsystem gain application. As a result of configuring the switch Q22 as analways-on switch, the second receiver coil L3 and the capacitor C2 forma half-bridge rectifier. Such a half-bridge rectifier helps to increasethe gain of the receiver.

In the first phase of the first configuration of the receiver, theswitches Q11, Q14, Q21 and Q23 are turned off as indicated by the arrowson the respective symbols. The switches Q12, Q13, Q22 and Q24 are turnedon. As indicated by the dashed line shown in FIG. 3 , the current flowsthrough the switch Q24, the receiver capacitor C2, the receiver coil L2and the switch Q22. The current is used to charge the receiver capacitorC2. The voltage across the receiver capacitor C2 is approximately equalto the voltage across the receiver coil L2.

FIG. 4 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a second phase of the first configuration ofthe receiver in the high system gain application in accordance withvarious embodiments of the present disclosure. In the second phase ofthe first configuration of the receiver, the switches Q12, Q13, Q21 andQ24 are turned off as indicated by the arrows on the respective symbols.The switches Q11, Q14, Q22 and Q23 are turned on. As indicated by thedashed line shown in FIG. 4 , the current flows through the switch Q22,the receiver coil L2, the receiver capacitor C2 and the switch Q23. Asum of the voltage across the receiver coil L2 and the voltage acrossthe receiver capacitor C2 is applied to the output terminal Vo.

In some embodiments, in the first phase and the second phase discussedabove, the duty cycle of the transmitter circuit may be adjustable so asto adjust the voltage across the receiver capacitor C2. For example, byreducing the time of the first phase, the receiver capacitor C2 is notfully charged. As a result of having a short charge time, the voltageacross the receiver capacitor C2 is reduced accordingly. In other words,by controlling the time of the first phase, the voltage across thereceiver capacitor C2 is adjusted accordingly. In some embodiments,depending on different power levels of the wireless power transfersystem, the voltage across the receiver capacitor C2 is adjustedgradually so that the gain of the wireless power transfer system isinversely proportional to the power level of the wireless power transfersystem.

FIG. 5 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a first phase of the second configuration ofthe receiver in the high system gain application in accordance withvarious embodiments of the present disclosure. In the first phase of thesecond configuration of the receiver, the switches Q12, Q13, Q21 and Q23are turned off as indicated by the arrows on the respective symbols. Theswitches Q11, Q14, Q22 and Q24 are turned on. As indicated by the dashedline shown in FIG. 5 , the current flows through the switch Q22, thereceiver coil L2, the receiver capacitor C2 and the switch Q24. Thecurrent is used to charge the receiver capacitor C2. The voltage acrossthe receiver capacitor C2 is approximately equal to the voltage acrossthe receiver coil L2.

FIG. 6 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a second phase of the second configuration ofthe receiver in the high system gain application in accordance withvarious embodiments of the present disclosure. In the second phase ofthe second configuration of the receiver, the switches Q11, Q14, Q22 andQ23 are turned off as indicated by the arrows on the respective symbols.The switches Q12, Q13, Q21 and Q24 are turned on. As indicated by thedashed line shown in FIG. 6 , the current flows through the switch Q24,the receiver capacitor C2, the receiver coil L2 and the switch Q21. Asum of the voltage across the receiver coil L2 and the voltage acrossthe receiver capacitor C2 is applied to the output terminal Vo.

FIG. 7 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a first phase of the low system gainapplication in accordance with various embodiments of the presentdisclosure. In the low system gain application, the transmitter is ahigh power transmitter. For example, the power transferred between thetransmitter and the receiver is in a range from about 40 W to about 80W. The input voltage of the transmitter is about 20 V. In order to becompatible with the high power transmitter, the gain of the receiver hasto be reduced accordingly. The rectifier circuit is configured as afull-bridge rectifier to lower the gain of the wireless power system.

In the first phase of the low system gain application, the switches Q11,Q14, Q21 and Q24 are turned off as indicated by the arrows on therespective symbols. The switches Q12, Q13, Q22 and Q23 are turned on.The current flows through the switch Q22, the receiver coil L2, thereceiver capacitor C2 and the switch Q23 as indicated by the dashed lineshown in FIG. 7 .

FIG. 8 illustrates a schematic diagram of the rectifier circuitconfigured to operate in a second phase of the low system gainapplication in accordance with various embodiments of the presentdisclosure. In the second phase of the low system gain application, theswitches Q12, Q13, Q22 and Q23 are turned off as indicated by the arrowson the respective symbols. The switches Q11, Q14, Q21 and Q24 are turnedon. The current flows through the switch Q24, the receiver capacitor C2,the receiver coil L2 and the switch Q21 as indicated by the dashed lineshown in FIG. 8 .

One advantageous feature of having the low system gain applicationdescribed above with respect to FIGS. 7-8 is the receiver shown in FIG.2 is able to achieve high efficiency. In particular, the inductance ofthe receiver coil L2 is small in comparison with the inductance of thetransmitter coil L1. Such a small inductance coil has low resistance.The low resistance helps to reduce the coil temperature, therebyimproving the efficiency of the wireless power transfer system.

FIG. 9 illustrates a flow chart of controlling the receiver shown inFIG. 2 in accordance with various embodiments of the present disclosure.This flowchart shown in FIG. 9 is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Forexample, various steps illustrated in FIG. 9 may be added, removed,replaced, rearranged and repeated.

Referring back to FIG. 1 , the wireless power transfer system comprisesa transmitter and a receiver. Depending on different applications, thetransmitter may be a high power transmitter. When the receiver (e.g.,receiver shown in FIG. 2 ) is magnetically coupled to this high powertransmitter, the receiver is configured to operate in a low system gainapplication. On the other hand, when the transmitter is a low powertransmitter, the receiver is configured to operate in a high system gainapplication.

The receiver comprises a rectifier circuit. Depending on differentsystem gain applications, the rectifier circuit may be configureddifferently so that the receiver is compatible with differentapplications. The receiver is controlled according to the followingsteps.

At step 902, a controller is configured to determine a system gain ofthe wireless power transfer system. The wireless power transfer systemcomprises a transmitter coil, a receiver coil and a rectifier circuitcoupled to the receiver coil. The wireless power transfer system maytransfer a small amount of power, which requires a normal gain betweenthe transmitter and the receiver. On the other hand, the wireless powertransfer system may transfer a large amount of power, which requires areduced gain between the transmitter and the receiver.

At step 904, in response to a high system gain application of thewireless power transfer system, the rectifier circuit is configured as ahalf-bridge rectifier.

At step 906, in response to a low system gain application of thewireless power transfer system, the rectifier circuit is configured as afull-bridge rectifier.

Although embodiments of the present disclosure and its advantages havebeen described in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. An apparatus comprising: a receiver coilconfigured to be magnetically coupled to a transmitter coil; and arectifier circuit coupled to two terminals of the receiver coil,wherein: in response to a high system gain of the apparatus, therectifier circuit is configured as a half-bridge rectifier; and inresponse to a low system gain of the apparatus, the rectifier circuit isconfigured as a full-bridge rectifier, wherein a system gain of theapparatus is a ratio of an output voltage of a receiver to an inputvoltage of a transmitter, and wherein the transmitter and the receiverare coupled to each other through the transmitter coil and the receivercoil, and wherein the rectifier circuit comprises a first switch and asecond switch connected in series, and a third switch and a fourthswitch connected in series, and wherein in response to the high systemgain of the apparatus, in a first configuration of the high system gainof the apparatus, the second switch is configured as a first always-onswitch, and in a second configuration of the high system gain of theapparatus, the fourth switch is configured as a second always-on switch.2. The apparatus of claim 1, wherein: a common node of the first switchand the second switch is connected to a first terminal of the receivercoil; and a common node of the third switch and the fourth switch isconnected to a second terminal of the receiver coil through a receivercapacitor.
 3. The apparatus of claim 2, wherein: in response to the highsystem gain of the apparatus, when the second switch is configured asthe first always-on switch, a sum of a voltage across the receiver coiland a voltage across the receiver capacitor is applied to a load coupledto the apparatus through the second switch and the third switch.
 4. Theapparatus of claim 2, wherein: in response to the high system gain ofthe apparatus, when the fourth switch is configured as the secondalways-on switch, a sum of a voltage across the receiver coil and avoltage across the receiver capacitor is applied to a load coupled tothe apparatus through the first switch and the fourth switch.
 5. Theapparatus of claim 2, wherein: in response to the low system gain of theapparatus, the rectifier circuit is configured to operate in twodifferent phases.
 6. The apparatus of claim 5, wherein: in a first phaseof the two different phases, the first switch and the fourth switch areturned off, and the second switch and the third switch are turned on;and in a second phase of the two different phases, the first switch andthe fourth switch are turned on, and the second switch and the thirdswitch are turned off.
 7. The apparatus of claim 1, wherein: thereceiver coil is magnetically coupled to the transmitter coil fortransferring energy in a wireless power transfer system; and a powerlevel in response to the low system gain of the apparatus is higher thana power level in response to the high system gain of the apparatus. 8.The apparatus of claim 1, wherein: the rectifier circuit has outputscoupled to a load and inputs connected to the two terminals of thereceiver coil through a receiver capacitor, and wherein the rectifiercircuit is configured to convert an alternating polarity waveform into asingle polarity waveform, and wherein a voltage across the receivercapacitor is adjustable through adjusting a duty cycle of the apparatus.9. A method comprising: determining a system gain of a wireless powertransfer system comprising a transmitter coil, a receiver coil and arectifier circuit coupled to the receiver coil, wherein the system gainof the wireless power transfer system is a ratio of an output voltage ofa receiver to an input voltage of a transmitter, and wherein thetransmitter and the receiver are coupled to each other through thetransmitter coil and the receiver coil; in response to a high systemgain application of the wireless power transfer system, configuring therectifier circuit as a half-bridge rectifier; and in response to a lowsystem gain application of the wireless power transfer system,configuring the rectifier circuit as a full-bridge rectifier, wherein apower level of the low system gain application is higher than a powerlevel in the high system gain application.
 10. The method of claim 9,further comprising a receiver capacitor, wherein: the rectifier circuitcomprises a first switch and a second switch connected in series, and athird switch and a fourth switch connected in series, and wherein: acommon node of the first switch and the second switch is connected to afirst terminal of the receiver coil; and a common node of the thirdswitch and the fourth switch is connected to a second terminal of thereceiver coil through the receiver capacitor.
 11. The method of claim10, further comprising: in a first phase of the high system gainapplication, configuring the rectifier circuit such that a current flowsthrough the second switch, the receiver coil, the receiver capacitor andthe fourth switch; and in a second phase of the high system gainapplication, configuring the rectifier circuit such that the currentflows through the second switch, the receiver coil, the receivercapacitor and the third switch.
 12. The method of claim 10, furthercomprising: in a first phase of the high system gain application,configuring the rectifier circuit such that a current flows through thesecond switch, the receiver coil, the receiver capacitor and the fourthswitch; and in a second phase of the high system gain application,configuring the rectifier circuit such that the current flows throughthe fourth switch, the receiver capacitor, the receiver coil and thefirst switch.
 13. The method of claim 10, further comprising: in a firstphase of the low system gain application, configuring the rectifiercircuit such that a current flows through the second switch, thereceiver coil, the receiver capacitor and the third switch; and in asecond phase of the low system gain application, configuring therectifier circuit such that the current flows through the fourth switch,the receiver capacitor, the receiver coil and the first switch.
 14. Themethod of claim 10, further comprising: reducing a duty cycle of thewireless power transfer system so that the receiver capacitor is notfully charged and a voltage across the receiver capacitor is reducedthrough adjusting the duty cycle, and gradually adjusting the voltageacross the receiver capacitor so that a gain of the wireless powertransfer system is inversely proportional to a power level of thewireless power transfer system.
 15. The method of claim 9, wherein: inthe low system gain application, the system gain of the wireless powertransfer system is less than 0.8; and an inductance of the transmittercoil is greater than an inductance of the receiver coil.
 16. The methodof claim 9, wherein: the half-bridge rectifier functions as a voltagebooster.
 17. A system comprising: a transmitter coil coupled to an inputpower source through a transmitter circuit; a receiver coil magneticallycoupled to the transmitter coil; and a rectifier circuit coupled to thereceiver coil, wherein in response to a low input voltage and a highsystem gain of the system, the rectifier circuit is configured as ahalf-bridge rectifier to boost an output voltage of the system, whereina system gain of the system is a ratio of an output voltage of areceiver of the system to an input voltage of a transmitter of thesystem, and wherein the transmitter and the receiver are coupled to eachother through the transmitter coil and the receiver coil, and whereinthe rectifier circuit comprises a first switch and a second switchconnected in series, and a third switch and a fourth switch connected inseries, and wherein in response to the high system gain of the system,in a first configuration of the high system gain of the system, thesecond switch is configured as a first always-on switch, and in a secondconfiguration of the high system gain of the system, the fourth switchis configured as a second always-on switch.
 18. The system of claim 17,wherein: the rectifier circuit is coupled to the receiver coil through acapacitor.
 19. The system of claim 17, wherein: in response to a highinput voltage of the system, the rectifier circuit is configured as afull-bridge rectifier.
 20. The system of claim 19, wherein: in responseto the high input voltage of the system, the transmitter coil, thereceiver coil and the rectifier circuit form a low gain wireless powertransfer system.